US007792392B2
(i2) United States Patent
Chen et al.
(io) Patent No.: US 7,792,392 B2
(45) Date of Patent: Sep. 7, 2010
(54) FIBER OPTIC GAS SENSOR
(75) Inventors: Peng Chen, Wexford, PA (US); Michael
P. Buric, Pittsburgh, PA (US); Philip R.
Swinehart, Columbus, OH (US);
Mokhtar S. Maklad, Westerville, OH
(US)
(73) Assignee: University of Pittsburgh — Of the
Commonwealth System of Higher
Education, Pittsburgh, PA (US)
( * ) Notice: Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 164 days.
(21) Appl.No.: 11/957,746
(22) Filed: Dec. 17, 2007
(65) Prior Publication Data
US 2009/0129721 Al May 21, 2009
Related U.S. Application Data
(60) Provisional application No. 60/870,431, filed on Dec.
18,2006.
(30) Foreign Application Priority Data
Dec. 9, 2006 (DE) 10 2006 058 138
(51) Int.Cl.
G02B 6/00 (2006.01)
G02B 6/34 (2006.01)
(52) U.S. Cl 385/12; 385/13; 385/37
(58) Field of Classification Search 385/12,
385/13, 37
See application file for complete search history.
(56) References Cited
U.S. PATENT DOCUMENTS
3,927,555 A * 12/1975 Godwin et al 73/31.01
5,280,172 A * 1/1994 DiBinetal 250/227.21
6,185,344 Bl* 2/2001 Bevenotetal 385/12
6,897,960 B2 * 5/2005 DiMeo et al 356/437
7,521,252 B2* 4/2009 Carpenter et al 436/144
2003/0218124 Al 11/2003 Johnson etal.
(Continued)
OTHER PUBLICATIONS
“The characterization hydrogen sensors based on Palladium electro-
plated fiber Bragg gratings (FBG)”, by Peng et al, SPIE conference of
Sensory Phenomena and measurement Instrumentation for Smart
Structures and Materials, Mar. 1999, vol. 3670, pp. 42-53.*
(Continued)
Primary Examiner — Charlie Peng
Assistant Examiner — Robert Tavlykaev
(74) Attorney ; Agent, or Firm — Eckert Seamans Cherin &
Mellott, LLC; Philip E. Levy, Esq.
(57) ABSTRACT
A gas sensor includes an in-fiber resonant wavelength device
provided in a fiber core at a first location. The fiber propagates
a sensing light and a power light. A layer of a material is
attached to the fiber at the first location. The material is able
to absorb the gas at a temperature dependent gas absorption
rate. The power light is used to heat the material and increases
the gas absorption rate, thereby increasing sensor perfor-
mance, especially at low temperatures. Further, a method is
described of flash heating the gas sensor to absorb more of the
gas, allowing the sensor to cool, thereby locking in the gas
content of the sensor material, and taking the difference
between the starting and ending resonant wavelengths as an
indication of the concentration of the gas in the ambient
atmosphere.
12 Claims, 20 Drawing Sheets
US 7,792,392 B2
Page 2
U.S. PATENT DOCUMENTS
2004/0173004 A1 9/2004 Eblen, Jr. et al.
2005/0163424 Al * 7/2005 Chen 385/37
OTHER PUBLICATIONS
“Hydrogen sensor based on a palladium-coated fibre-taper with
improved time-response,” by Zalvidea et al , Sensors and Actuators B,
vol. 114, 2006, pp. 268-274, available online since Jul. 22, 2005.*
Voet et al., 17th International Conference on Optical Optical Fibre
Sensors, May 23-27, 2005, vol. 5855, 6 pp.
Lundstrom et al., “A Hydrogen-Sensitive MOS Field-effect Transis-
tor”, Applied Physics Letters, vol. 26, No. 2, Jan. 15, 1975 pp. 55-57.
Steele et al., “Palladium/Cadmium-Sulfide Schottky Diodes for
Hydrogen Detection”, Applied Physics Letters, vol. 28, No. 11, Jun.
1, 1976, pp. 687-688.
D’Amico et al., “Palladium-Surface Acoustic Wave Interaction for
Hydrogen Detection”, Appl. Phys. Lett. 41(3), Aug. 1, 1982, pp.
300-301.
Villatoro et al., “Fast Detection of Hydrogen with Nano Fiber Tapers
Coated with Ultra Thin Palladium Layers”, Optics Express, Jun. 27,
2005, vol. 13, No. 13, pp. 5087-5092.
Tabib-Azar et al., “Highly Sensitive Hydrogen Sensors Using Palla-
dium Coated Fier Optics with Exposed Cores and Evanescent Field
Interactions”, Sensors and Actuators B 56 (1999) 158-163.
Bevenot et al., “Surface Plasmon Resonance Hydrogen Sensor Using
an Optical Fibre”, Meas. Sci. Technol. 13 (2002) 118-124.
Bevenot et al., “Hydrogen Leak Detection using an Optical Fibre
Sensor for Aerospace Applications”, Sensors and Actuators B 67
(2000) 57-67.
M.A. Butler, “Optical Fiber Hydrogen Sensor”, Appl. Phys. Lett.
45(10), Nov. 15, 1984, pp. 1007-1009.
Favier et al., “Hydrogen Sensors and Swtiches from Electro deposited
Palladium Mesowire Arrays”, Science 193, 2227 (2001); DOI:
10. 1 126 science. 1063 189, pp. 2227-2231.
Sekimoto, “A Fiber-Optic Evanescent -Wave Hydrogen Gas Sensor
Using Palladium-Supported Tungsten Oxide”, Sensors and Actuators
B 66 (2000) 142-145.
* cited by examiner
FIG. 4B
U.S. Patent
Sep. 7, 2010
Sheet 3 of 20
US 7,792,392 B2
E
CQ
T5
CD
O
c
CO
■ 4 — ■
o
0
H—
0
DC
FIG. 5A
Wavelength (nm)
5
4
E 3
<1 2
1
0
0 30 60 90 120 150
Laser Power (mW)
FIG. 5B
U.S. Patent
Sep. 7, 2010
Sheet 4 of 20
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Wavelength (nm)
FIG. 6
1542 1544 1546 1548 1550 1552 1554
Wavelength (nm)
FIG. 7
U.S. Patent
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Sheet 5 of 20
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FIG. 8
FIG. 9
Wavelength (nm)
U.S. Patent
Sep. 7, 2010
Sheet 6 of 20
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FIG. 10B
170
U.S. Patent
Sep. 7, 2010
Sheet 7 of 20
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FIG. 11
Wavelength (nm)
1554 1556 1558
Resonance Wavelength (nm)
FIG. 12
<j.o. Patent
FIG. 13
Resonance Wa
FIG. 14
Flow Vi
3
U.S. Patent sep. i, 2010
FIG. 18
mtn\
T
T
K
»
Wavelength Shift (nm)
U.S. Patent
K
1
FIG. 21
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Sheet 14 of 20
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FIG. 23A
Wavelength (nm)
FIG. 23B
U.S. Patent
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Sheet 15 of 20
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FIG. 23C
FIG. 23D
U.S. Patent
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Sheet 16 of 20 US 7,792,392 B2
U.S. Patent
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Sheet 17 of 20
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FIG. 26
FBG Wavelength (nm)
Reflected Power (nW) 3 Reflected Power (nW)
U.S. Patent
Sep. 7, 2010
Sheet 19 of 20
US 7,792,392 B2
. 29 A
FIG. 29B
FBG shift, AX (nm)
1
FIBER OPTIC GAS SENSOR
US 7,792,392 B2
CROSS-REFERENCE TO RELATED
APPLICATION
5
This application claims the benefit of U.S. Provisional
Application No. 60/870,431 entitled “Active Fiber Bragg
Grating Hydrogen Sensors For All-Temperature Operation,”
which was filed on Dec. 18, 2006.
10
GOVERNMENT CONTRACT
This work was supported in part by a grant from NASA
under SBIR Contract No. NNC06CA52C. The United States
government may have certain rights in the invention 15
described herein.
BACKGROUND OF THE INVENTION
1 . Field of the Invention 20
The present invention relates to sensors for sensing the
presence of gasses such as hydrogen, and more specifically to
fiber optic gas sensor that employs an in- fiber resonant wave-
length device, such as an FBG, wherein performance is 25
improved using an in-fiber power light.
2. Description of Related Art
Fiber optic components, such as, without limitation, Fiber
Bragg Gratings (FBGs), fiber interferometers, and Fabry-
Perot cavities (FPs) are well known and are key components 30
used in many optical communication and sensing applica-
tions. For example, such components are often utilized in
constructing multiplexers and de-multiplexers used in wave-
length division multiplexing (WDM) optical communica-
tions systems, and in constructing optical strain sensors, tern- 35
perature sensors, pressure or vibration sensors, chemical
sensors and accelerometers. In-fiber optic components,
meaning those provided in or as part of an optical fiber, offer
several important advantages over other optical and elec-
tronic devices, including low manufacturing cost, immunity 40
to electromagnetic radiation and changing (often harsh)
ambient conditions, an explosive-proof and in-vivo safe
nature, long lifetime, and high sensitivity.
Historically, in-fiber optic components have been passive,
meaning they cannot be actively adjusted and/ or reconfigured 45
once deployed to, for example, adopt new network topologies
or adjust sensing parameters including sensitivity, set point,
triggering time, dynamic range and responsivity. In addition,
passive in-fiber optic components require delicate and costly
packaging to eliminate temperature drifting. These facts 50
have, despite the advantages described above, limited the
performance and use of in-fiber components. As a result,
work has been done to develop tunable in-fiber optic compo-
nents, such as a tunable FBG. As is known in the art, an FBG
consists of a series of perturbations, forming a grating from 55
periodic variations in the index of refraction along the length
of an optical fiber, that will be here termed “grating ele-
ments”. An FBG reflects a spectral peak of a light back
through the fiber toward the light source, and the particular
spectral peak (called the resonance wavelength) that is 60
reflected depends upon the grating spacing. A corresponding
valley is transmitted forward though the fiber. Thus, changes
in the length of the fiber due to heat, tension or compression
will change the spacing of the grating index of refraction
variations (and to a lesser extent, the grating component indi- 65
ces of refraction) and thus the wavelength of the light that is
reflected.
2
A typical prior art implementation of an FBG is shown in
FIG. 1, and includes optical fiber 5 having core 10 surrounded
by cladding 15 , wherein the core 10 is provided with a grating
20 . The light transmitted through optical fiber 5 and reflected
by grating 20 is shown by the arrow in FIG. 1 . The grating 20
shown in FIG. 1 has a constant period, A, meaning the grating
elements are evenly spaced, and is referred to as a uniform
FBG. FBGs may also include gratings that have a varying
period. Such FBGs are referred to as chirped FGBs, and
reflect multiple spectral peaks or a wide spectrum of light.
Long period gratings, in which the spacing is large compared
to the core diameter, and apodized gratings are also useful.
Tuning mechanisms (for changing the fiber length and other
characteristics such as refractive index) that have been previ-
ously explored for FBGs and other in-fiber optic components
include on-fiber electrical heating, piezoelectric actuators,
mechanical stretching and bending, and acoustic modulation.
The problem has been that each of these tuning mechanisms
requires an energy source for operation, which, to date, has
been electrical. In particular, electrical cable must be run with
the optical fiber to provide current for on-fiber heating ele-
ments, to supply voltages to drive piezoelectric actuators, to
drive stepper motors to stretch and bend the fibers, or to
initialize acoustic waves. Additional cabling of this sort is
problematic, as it, among other things, typically increases
manufacturing costs, is bulky, is not immune to electromag-
netic radiation, is difficult to embed in materials and struc-
tures, and typically has a shorter lifetime than the associated,
normally durable optical fibers.
Thus, there is a need for a mechanism for powering and
tuning in-fiber optic components that does not require addi-
tional electrical cabling. Such a mechanism would allow fiber
optic systems to take advantage of the improved performance
and functionality of in-fiber optic components without the
disadvantages and drawbacks presented by electrical cabling.
Moreover, hydrogen is becoming an attractive alternative
fuel source for use in clean-burning engines and power plants.
Some mission-critical applications such as the Space Shuttle
engine already employ liquid hydrogen as a fuel. Unfortu-
nately, the use of highly flammable liquid H 2 also introduces
a number of safety concerns due to its rapid evaporation rate
and low explosive limit. In order to mitigate the high risk of
explosion due to leaks in hydrogen fueled systems, an effi-
cient system of H 2 leak detection is needed. Such a system
should allow detection well below the 4% mass concentration
explosion limit of hydrogen.
Recently, various electrical sensors based on the change of
resistivity of palladium (Pd) have been developed including
some nano-scale devices. Examples of such sensors are
described in A. D’Amico et al, “Palladium-surface acoustic
wave interaction for hydrogen detection,” Appl. Phys. Lett.,
vol. 41, pp. 300-301, (1982); I. Lundstrom et al., “A hydro-
gen-sensitive MOS field-effect transistor,” Appl. Phys. Lett.,
vol. 26, pp. 55-57, (1975); andM. C. Steele etal., “Palladium/
cadmium- sulfide Schottky diodes for hydrogen detection,”
Appl. Phys. Lett. vol. 28, pp. 687-688, (1976). Furthermore,
because of their explosion proof nature, the desirability of
fiber optic sensors has been recognized in recent years and
more emphasis has been placed on the development of optical
sensors such as those described in M. Tabib-Azar et al.,
“Highly sensitive hydrogen sensors using palladium coated
fiber optics with exposed cores and evanescent field interac-
tions,” Sens. Act., vol. B 56, pp. 158-163, (1999); J. Villatoro
et al., “Fast detection of hydrogen with nano fiber tapers
coated with ultra thin palladium layers,” Optics. Exp., vol. 13,
pp. 5087-5092, (2005); J. Villatoro et al, “In-Line Highly
Sensitive Hydrogen Sensor Based on Palladium-Coated
US 7,792,392 B2
3
Single-Mode Tapered Fibers,” IEEE Sens. Journal, vol. 3, pp.
533-537 (2003); and X. Bevenot et al., “Surface plasmon
resonance hydrogen sensor using an optical fibre,” IOP Meas.
Sci. Technol., vol. 13, pp. 118-124, (2002). Of particular
interest are optical sensors that are of the type that can be
interrogated remotely over long fibers, such as those
described in X. Bevenot et al., “Hydrogen leak detection
using an optical fibre sensor for aerospace applications,”
Sens. Act., vol. B 67, pp. 57-67, (2000); A. Trouillet et al.,
“Fibre gratings for hydrogen sensing,” Measurement Science
& Technol., vol. 17(5), pp. 1124-1128, (2006); and J. A.
Guemes et al., “Comparison of three types of fibre optic
hydrogen sensors within the frame of CryoFOS project,”
Third International Conference on Experimental Mechanics
and Third Conference of the Asian Committee on Experimen-
tal Mechanics, Proceedings of the SPIE, Vol. 5855, pp. 1000-
1003 (2005). Another significant advantage of fiber-based
hydrogen sensors is the capability of providing numerous
sensing points in order to generate data regarding the location
of the leak itself.
One of the most important requirements for any leak detec-
tion system, particularly one for detecting hydrogen leaks, is
the ability to operate over a large range of temperatures (e.g.,
for use near extremely cold liquid-H 2 tanks and pipes, as well
as in much warmer environments). In addition, with any
leak-detection system, response time is paramount to suc-
cessfully averting disaster. Although a number of sensing
solutions have been developed based on the use of a Pd-
coating as described above, those solutions share a common
problem, namely, due to palladium’s slow hydrogen absorp-
tion rate at low temperature (e.g., on the order of 20 degrees
C. and lower), the sensors exhibit an extremely low sensitivity
and slow response time at low temperatures. Thus, there is a
need for a fiber optic sensing solution that exhibits improved
sensitivity and response time at low temperatures.
SUMMARY OF THE INVENTION
The present invention, in one embodiment, provides a sen-
sor for sensing a gas, such as hydrogen, that includes an
optical fiber having a core and a wavelength resonant in-fiber
optic component, such as a fiber Bragg grating or Fabry -Perot
filter, provided in the core at a first location. The optical fiber
propagates a sensing light and a power light, with the sensing
light being propagated in the core. The wavelength resonant
in-fiber optic component receives the sensing light and
reflects a reflected light having a resonance wavelength that is
dependent on a characteristic, such as a grating spacing, of the
wavelength resonant in-fiber optic component. The reflected
light is used to determine at least one of a presence of and a
concentration of the gas. In addition, at least one layer of a
certain material is attached to the optical fiber in proximity to
the first location. In particular, the material is a material, such
as palladium or a palladium alloy, that is able to absorb the gas
being sensed (e.g., hydrogen) at a temperature dependent gas
absorption rate that increases when a temperature of the mate-
rial is increased up to some limiting high temperature that is
dependent on environmental conditions. The material
induces a strain in the optical fiber when the material absorbs
the gas, with a magnitude of the strain being dependent upon
an amount of the gas that is absorbed. The strain in the optical
fiber changes the characteristic, e.g., grating spacing. Finally,
the optical fiber is structured to allow at least a portion of the
power light to be used to heat the material. In one particular
embodiment, the optical fiber is structured to allow at least a
portion of the power light to be released from the optical fiber
at the first location and be absorbed by the material. In another
4
embodiment, at least a portion of the power light may be
absorbed by another portion of the fiber, such as the inner or
out cladding, generate heat that in turn heats the material. The
absorbed power light heats the material and increases the gas
5 absorption rate. As a result, the sensitivity and response time
of the sensor is improved, particularly at low temperatures.
In another embodiment, the invention provides a method of
sensing a gas, such as hydrogen, that includes providing an
optical fiber, wherein the optical fiber has a core, a wave-
10 length resonant in-fiber optic component, such as a fiber
Bragg grating or a Fabry-Perot filter, provided in the core at a
first location, and at least one layer of a material, such as
palladium or an alloy of palladium, attached to the optical
fiber in proximity to the first location. The material is able to
1 5 absorb the gas at a temperature dependent gas absorption rate
that increases when the temperature of the material is
increased. The material induces a strain in the optical fiber
when the material absorbs the gas, with a magnitude of the
strain being dependent upon an amount of the gas that is
20 absorbed by the material. The strain changes a characteristic,
e.g., the grating spacing, of the wavelength resonant in-fiber
optic component. The method further includes propagating a
sensing light in the core, wherein the wavelength resonant
in-fiber optic component receives the sensing light and
25 reflects a reflected light having a resonance wavelength that i s
dependent on the characteristic of the wavelength resonant
in-fiber optic component. The method still further includes
propagating a power light in the optical fiber, and using the
power light to directly or indirectly heat the material and
30 therefore increase the gas absorption rate. This may include
causing at least a portion of the power light to be released
from the optical fiber at the first location and be absorbed by
the material. The absorbed power light heats the material and
increases the gas absorption rate. Finally, the method includes
35 using the reflected light to determine at least one of a presence
of and a concentration of the gas.
In yet another embodiment, the invention provides a
method of sensing a gas at an ambient temperature that
includes providing an optical fiber, wherein the optical fiber
40 has a core, a wavelength resonant in-fiber optic component
provided in the core at a first location, and at least one layer of
a material attached to the optical fiber in proximity to the first
location. The material is able to absorb the gas at a tempera-
ture dependent gas absorption rate that increases when a
45 temperature of the material is increased. The method further
includes propagating a first sensing light in the core at the
ambient temperature, wherein the wavelength resonant in-
fiber optic component receives the first sensing light and
reflects a first reflected light having a first resonance wave-
50 length that is dependent on an ambient characteristic of the
wavelength resonant in-fiber optic component. Also, the
method includes propagating a power light in the optical fiber
for a defined period of time during which at least a portion of
the power light is used to directly or indirectly heat the mate-
55 rial. In one embodiment, at least a portion of the power light
is released from the optical fiber at the first location and
absorbed by the material, wherein the absorbed power light
heats the material. Alternatively, the power light may be
absorbed by another portion of the fiber, such as a cladding
60 layer or even the core, which in turn causes heat to be gener-
ated which heats the material. The heating of the material
causes the material to induce a strain in the optical fiber that
changes the ambient characteristic to a changed characteris-
tic. The method still further includes allowing the material to
65 cool to a temperature substantially equal to the ambient tem-
perature after the defined period of time has expired, and
propagating a second sensing light in the core after the mate-
US 7,792,392 B2
5
rial is allowed to cool, wherein the wavelength resonant in-
fiber optic component receives the second sensing light and
reflects a second reflected light having a second resonance
wavelength that is dependent on the changed characteristic.
Finally, the method includes determining a difference
between the first resonance wavelength and the second reso-
nance wavelength, and using the difference to determine at
least one of a presence of and a concentration of the gas.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages of the present invention will
become readily apparent upon consideration of the following
detailed description and attached drawings, wherein:
FIG. 1 is a side view of a prior art optical fiber including a
Fiber Bragg Grating;
FIGS. 2 A and 2B are a partial cross-sectional side view and
a side view, respectively, of an optical fiber having a tunable
in-fiber optic component according to one embodiment of the
present invention;
FIG. 3 is a cross-sectional end view of the optical fiber
shown in FIGS. 2 A and 2B;
FIGS. 4 A and 4B are a partial cross-sectional side view and
a side view, respectively, of an optical fiber having a tunable
in-fiber optic component according to an alternate embodi-
ment of the present invention;
FIG. 5A is a graph illustrating a reflection spectrum shift
and FIG. 5B is a graph illustrating a spectrum shift as a
function of power light of a particular implementation of the
optical fiber shown in FIGS. 4A and 4B;
FIG. 6 is a graph illustrating a reflection spectrum expan-
sion of a particular implementation of the optical fiber shown
in FIGS. 4A and 4B;
FIG. 7 is a graph illustrating a reflection spectrum com-
pression of a particular implementation of the optical fiber
shown in FIGS. 4 A and 4B;
FIG. 8 is a partial cross-sectional side view of an optical
fiber having a tunable in-fiber optic component according to
a further alternate embodiment of the present invention;
FIG. 9 is a graph illustrating the notch filter characteristics
of an implementation of the optical fiber shown in FIG. 8;
FIGS. 10A and 10B are a partial cross-sectional side view
and a side view, respectively, of a fiber optic system according
to a further alternate embodiment of the present invention;
FIG. 11 is a graph illustrating a spectrum shift of an imple-
mentation of the fiber optic system shown in FIGS. 10A and
10B;
FIG. 12 is a graph illustrating operation of a fluid flow
sensor according to an aspect of the present invention;
FIG. 13 is a graph illustrating operation of a flow sensor
according to another aspect of the present invention;
FIG. 14 is a graph that shows the resonance wavelength
shifts as a function of fluid flow velocity of the flow sensor the
operation of which is demonstrated in FIG. 13;
FIG. 15 is a graph that shows the spectral evolution of a
particular flow sensor according to the present invention;
FIG. 16 is a graph that shows the spectral separation pro-
duced by a particular fluid flow sensor according to the
present invention;
FIGS. 17 and 18 are graphs that demonstrate the operation
of a particular fluid flow sensor according to the present
invention in a constant wavelength (variable power) mode.
FIG. 19 is a schematic diagram of a tunable (active) optical
fiber system including an FBG type in-fiber optic component
powered by in-fiber light that may be utilized as a liquid level
sensor according to a further aspect of the present invention;
6
FIG. 20 is a graph that shows the spectral response of the
FBG of the optical fiber system of FIG. 19 under certain
conditions;
FIG. 21 is a graph that show the thermal responses of a
5 heated grating, such as the FBG of optical fiber system shown
in FIG. 19, in air, water, and liquid nitrogen at atmospheric
pressure as a function of input laser power (power light)
according to an aspect of the present invention;
FIG. 22 is a schematic diagram of an alternative tunable
to (active) optical fiber system including multiple FBG type
in- fiber optic components powered by in-fiber light that may
be utilized as a liquid level sensor according to a further
aspect of the present invention;
FIGS. 23A through 23D show the reflection spectrum of
1 5 each of the FBGs of the optical fiber system of FIG. 22 under
various conditions;
FIG. 24 is a side isometric view (in partial cross-section) of
a hydrogen sensor according to a further embodiment of the
present invention;
20 FIG. 25 is a side isometric view (in partial cross-section) of
an alternative hydrogen sensor according to a further embodi-
ment of the present invention;
FIG. 26 is a flowchart of a flash heating method according
to another embodiment of the invention that may be employed
25 with the hydrogen sensors shown in FIGS. 24 and 25 to
improve performance at low temperatures; and
FIG. 27-30 are graphs demonstrating experimental results
obtained by the inventors.
30 DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
The present invention relates to various systems and meth-
ods for providing active in-fiber optic components that are
35 powered by in-fiber light. Specifically, as described in greater
detail herein, various optical fibers are provided that propa-
gate both a sensing or signal light and a power light wherein
the power light is used to provide the energy required to tune
the in-fiber optic component.
40 FIGS. 2A and 2B are side views (2A in partial cross-
section) and FIG. 3 is a cross-sectional end view of optical
fiber 30 according to one embodiment of the present inven-
tion. As seen most readily in FIG. 3, optical fiber 30 includes
a core 35, inner cladding 40, outer cladding 45 and protective
45 layer 50. Preferably, core 35, inner cladding 40 and outer
cladding 45 are made of light propagating materials, wherein
core 35 has an index of refraction that is greater than the index
of refraction of inner cladding 4 0 , which in turn i s greater than
the index of refraction of outer cladding 45. Except as other-
50 wise described herein, establishing the relative indices of
refraction in this manner causes light propagating in core 35
to be confined therein, and light propagating in inner cladding
40 to be confined therein. Inner cladding 40 and outer clad-
ding 45 may be formed by a number of known techniques,
55 such as modified chemical vapor deposition (MCVD). The
thickness of outer cladding 45 is preferably about 10 jam to
allow for convenient optical tap region fabrication as
described elsewhere herein. In addition, inner cladding 40
and outer cladding 45 should be highly transparent.
60 In one particular embodiment, core 35 is made of a glass
material such as fused silica that is doped with germanium
and/or boron to increase the index of refraction thereof, inner
cladding 40 is made of fused silica, and outer cladding 45 is
made of fused silica that is doped with fluorine (preferably
65 3 -mole % fluorine) to decrease the index of refraction thereof.
Preferably, core 35 is doped to achieve anN.A. value of about
0.16, which is slightly higher than the N.A. value of 0.13 for
US 7,792,392 B2
7
standard telecommunications fibers. Both pure silica and
fluorine-doped silica are transparent to UV light, which
allows uniform penetration of KrF 248 nm laser radiation for
in-fiber FBG fabrication. In one particular example of this
particular embodiment, core 35 has an index of refraction 5
equal to about 1 .45, inner cladding 40 has an index of refrac-
tion equal to about 1 .445, and outer cladding 45 has an index
of refraction equal to about 1.44. In addition, in another
specific example, core 35 has an elliptical shape with an 8 (im
major axis and a 5 pm minor axis, inner cladding 40 has an to
outer diameter equal to about 105 pm, and outer cladding 45
has an outer diameter equal to about 125 pm. Alternatively,
outer cladding 45 may be made of a polymer such as clear
silicone or PFA. Preferably, core 35 is a polarization-main-
taining ellipse with a beat length ofless than 4 mm at 1550nm 15
to ensure high sensing sensitivity and to reduce polarization
phase noise. Other core and cladding sizes and shapes are
possible without limitation.
Referring to FIGS. 2A and 2B, optical fiber 30 includes an
in-fiber optic component 55 such as, for example, an FBG 20
(shown in FIG. 2A). In addition, an optical transducing ele-
ment is located in a position that is proximate to a portion of
optical fiber 30. In the embodiment shown in FIGS. 2 A and
2B, the optical transducing element is a light absorbing ther-
mal coating 60 provided around at least a portion of and 25
preferably the entirety of the outer circumference of optical
fiber 30 (FIG. 2 A shows thermal coating 60 in partial cut-
away to enable viewing of the other components of optical
fiber 30). According to one particular embodiment, thermal
coating 60 is a metal film, such as, for example, a silver, 30
nickel, titanium or other light absorbing metal film, that is
deposited on the outside of optical fiber 30 (protective layer
50 is removed at this location for reasons that will be clear
below and thermal coating 60 is applied to outer cladding 45)
by any one of many known coating methods such as plating, 35
sputtering and e-beam thermal evaporation. The thickness of
thermal coating 60 is preferably on the order of about 10 nm
to tens of microns. A key characteristic of thermal coating 60
is that is heats up and radiates and/or conducts heat when
exposed to certain types of light from inside optical fiber 30. 40
Other suitable materials such as, without limitation, light
absorbing polymers, carbon, semiconductors, ceramics, light
absorbing doped glasses, metal films of any kind, metal
oxides, metal nitrides, and metal carbides, may be used for
thermal coating 60. 45
Referring to FIG. 2A, optical fiber 30 also includes an
optical tap region 65 located in a portion of optical fiber 30
that is proximate to thermal coating 60. Optical tap region 65
is a region of optical fiber 30 that will allow certain light, as
described in greater detail below, that is propagating through 50
optical fiber 30 to leak out of (i.e., be released from) optical
fiber 30 and be absorbed by thermal coating 60. Optical tap
region 65 may be created in a number of ways. For example,
laser techniques or ion-implantation techniques may be used
to, in effect, damage inner cladding 40 in a selected region and 55
thereby alter its index of refraction such that the power light
75 will leak out of inner cladding 40 at optical tap region 65.
It is estimated that an index change of about 5x1 0 -3 to 1 xl 0 -2
through outer cladding 45 will be sufficient to release the
power light 75. 60
Preferably, a combination of deep UV laser radiation and
ion implantation are used to fabricate optical tap region 65.
Compaction produced by deep UV laser radiation in germa-
nium doped cores such as core 35 produces stress in the
core-cladding interface, which eventually damages the inter- 65
face region and produces leaking light. For example, a com-
bination of 157 nm F 2 vacuum UV lasers and 248 nm KrF
8
deep UV lasers based on type II photosensitivity response
may be used to fabricate long-period grating type optical taps.
KrF lasers are well suited for fabrication of uniform optical
taps for optical tap region 65 due to the relatively weak
absorption of such laser light by germanosilica waveguide
cores such as core 35. The type II photosensitivity can be
enhanced by using known hydrogen loading techniques. In
addition, anisotropic optical taps for optical tap region 65
with highly directional leaking light can readily be fabricated
with 157 nm F 2 laser radiation. Anisotropic optical taps are
convenient for leaking light collection and refocusing. For
example, highly anisotropic diffused light can be easily line-
focused to generate acoustic waves for active ultrasonic sens-
ing.
Furthermore, using an amplitude mask, the pulse fluence of
the optical tap fabricating laser(s) can be tailored along fiber
30 to achieve a uniform leaking light intensity. Angular uni-
formity of an optical tap fabricated by deep UV laser radiation
can be obtained by rotating fiber 3 0 during laser exposure. By
adjusting pulse fluence and accumulated fluence of the lasers,
optical taps can be fabricated with desired tap lengths, leaking
percentages, and emitting directionalities.
With respect to ion implantation, the magnitude and loca-
tion of an index of refraction change in a fiber can be precisely
controlled by the selected ion species, ion energy, and total
ion dose. As such, the optical damage can be localized
between the surface and the interface of the inner cladding 40
and the outer cladding 45. It has been learned, for example,
that 2 1 -MeV Si and 1 2-MeV C ions produce uniform vacancy
profiles and thus uniform index profiles, while) 0.8-MeV H
ions produce vacancies that are concentrated at the end of the
implantation trajectory.
Alternatively, blazed Bragg gratings may be used to imple-
ment optical tap region 65 by providing a blazed grating in a
core of a fiber such as core 35 shown in FIGS. 2A and 2B or
in the core of a single mode fiber similar to single mode fiber
portion 85 in a location that is near the thermal coating such
as thermal coating 60 or 115. In such an application, both the
power light and the sensing/signal light are propagated
through the same core. As is known in the art, blazed Bragg
gratings are fiber gratings that have grating planes that are at
an angle (<90 degrees) with respect to the longitudinal axis of
the fiber in which they are created. The angled nature of the
gratings causes the light reflected by the blazed Bragg grating
(the resonance wavelength) to be reflected at an angle with
respect to the longitudinal axis of the fiber. This light (the
power light) will be coupled out of the fiber core, into the
surrounding cladding, and out of the fiber, where it may be
absorbed by a thermal coating such as thermal coating 60 or
115. The particular blazing angles and the degree of change of
the index of refraction will determine the out-coupling effi-
ciency, and the period of the grating will determine the out-
coupling wavelength.
In operation, as illustrated in FIGS. 2A and 2B, a sensing
light 70 is directed through and propagates through core 35.
Sensing light 70 may be, for example, 1300 to 1700 nm light
generated by a diode laser, such as a swept tunable laser, or a
broadband source. Although the term sensing light is used
herein for illustrative purposes, it will be appreciated that
sensing light 70 may also be a signal propagating light used
in, for example, a fiber optic communication system, and the
term sensing light or sensing/signal light is intended to
include signal propagating or similar lights. As is known in
the art and as described elsewhere herein, sensing light 70 will
propagate through core 35 and encounter in-fiber optic com-
ponent 55, which, in the embodiment shown in FIG. 2 A, is an
FBG, and a particular resonance wavelength will be reflected
US 7,792,392 B2
9
back in the opposite direction. This functionality is essen-
tially the same as described in connection with the prior art
FBG shown in FIG. 1. According to an aspect of the present
invention, power light 75 is simultaneously directed through
both core 35 and inner cladding 40 (although power light 75
is shown propagating in a direction opposite the sensing light
70, it may also be propagated in the same direction as sensing
light 70. Power light 75 may be, for example, light generated
from a high-power diode laser array (not shown). Preferably,
power light is on the order of 0.1 to hundreds of watts with a
wavelength of between 600 mn and 1 600 nm. Power light 75
will, as seen in FIGS. 2 A and 2B, propagate through core 35
and inner cladding 40 and will be confined therein until it
reaches optical tap region 65. When power light 75 reaches
optical tap region 65, at least a portion of power light 75 will
leak out of inner cladding 40 and into outer cladding 45. The
portion of power light 75 that has leaked into outer cladding
45 will then be transmitted substantially radially outwardly
therefrom and will ultimately be absorbed by thermal coating
60. The absorption of power light 75 will cause the tempera-
ture of thermal coating 60 to increase. As a result, thermal
coating 60 will then radiate heat that is transmitted/conducted
through outer cladding 45 and inner cladding 40 and into core
35. The heat in core 35 heats the in- fiber optic component 55.
As is known, this heat will change (increase) the index or
indexes of refraction of in-fiber optic component 55 and will,
to an extent, change the size of (make larger) optical fiber 30,
each of which will alter the characteristics of in-fiber optic
component 55. In the case of an FBG as shown in FIG. 2 A,
these changes, resulting from the power light 75, will alter the
resonance wavelength of the FBG. Thus, power light 75 may
be used to power and tune the in-fiber optic component 55
provided in optical fiber 30. As will be appreciated, the inten-
sity and/or duration of power light 75 may be controlled to
selectively heat thermal coating 60 to produce particular
changes in the in-fiber optic component 55 (e.g., particular
resonance wavelengths).
FIGS. 4 A and 4B are side views (4A in partial cross-
section) of optical fiber 80 according to an alternate embodi-
ment of the present invention. Optical fiber 80 includes single
mode fiber portion 85 having core 90 and cladding 95 that is
joined to, such as by fusion splicing, multi -mode fiber portion
100 having core 105 and cladding 110. Single mode fiber
portion 85 may be any known, commercially available single
mode optical fiber material, and multi -mode fiber portion 100
may be any known, commercially available multi-mode opti-
cal fiber material. Optical fiber 80 includes an optical trans-
ducing element in the form of thermal coating 115 which is
similar in structure to thermal coating 60 shown in FIGS. 2A
and 2B. Optical fiber 80 also includes in-fiber optic compo-
nent 120, which, in the embodiment shown in FIGS. 4A and
4B, is a uniform FBG, but may also be other types of optic
components as described herein. A sensing light 125 is
directed through core 90 as shown. Single mode fiber portion
85 and multi -mode fiber portion 100 are joined to one another
at junction 130. As shown in FIGS. 4A and 4B, junction 130,
and in particular the differing diameters of core 90 and core
105, acts as an optical tap region that allows power light 135
that is directed through and propagates through core 105 to
leak out of core 105 and into cladding 95, where it is ulti-
mately absorbed by thermal coating 115. As described in
connection with FIGS. 2A and 2B, the absorbed power light
135 heats thermal coating 115 which in turn radiates heat that
is conducted therefrom and heats in-fiber optic component
120, thereby changing the operating characteristics thereof.
In one particular implementation investigated by the inven-
tor, in-fiber optical component 120 consisted of several 4 mm
10
long and 4 cm long uniform FBGs and several 4 cm long
linear chirped FBGs (1 nm/cm) written into single mode fiber
portion 85, which consisted of an SMF-28 fiber available
from Coming Incorporated of Corning, N.Y. Single mode
5 fiber portion 85 in this implementation consisted of a 1 25 jam
diameter (9 pm core) fiber, and multi-mode fiber portion 100
consisted of a 140 pm diameter (100 pm core) fiber. Thermal
coating 115 consisted of a 2 pm thick silver film that was
positioned approximately 5 mm from junction 130. Ten watts
10 of 9 1 0 nm laser light from a high power diode laser array was
coupled into the end of multi-mode fiber portion 100 using a
pair of 20x microscope objectives. The 910 mn light (power
light 135) propagated through core 105 of multi -mode fiber
portion 100 and leaked into cladding 95 of single mode fiber
15 portion 85 through junction 130. The leaking power light 135
was absorbed by thermal coating 115 and raised the tempera-
ture of in-fiber optic component 120. The reflection spectra of
the in-fiber optic component 120 was monitored with an
optical spectrum analyzer. FIG. 5A shows a spectrum evolu-
20 tion of a 4 mm uniform FBG forming part of in-fiber optic
component 120 heated with the power light 135 as just
described. It was estimated that approximately 130 mW of
power light 135 was leaked through to thermal coating 115,
raising the temperature of in-fiber optic component 120 sig-
25 nificantly and shifting the resonance wavelength as shown in
FIG. 5A. FIG. 5B shows the resonance wavelength shift as a
function of the input diode laser power (power light 135).
In addition, as will be appreciated, the energy of power
light 135 leaking out of core 150 and being transmitted
30 through cladding 95 falls exponentially with distance (assum-
ing a constant loss coefficient). Thus, a temperature gradient
is created along the length of thermal coating 115. In some
cases, this temperature gradient is longer than the FBGs form-
ing a part of in-fiber optic component 120. This gradient
35 modifies the spectrum response of the FBGs and may be used
to control the grating chirp and cause a spectrum stretch of the
4 cm long uniform FBG forming a part of in-fiber optic
component 120. FIG. 6 shows a spectrum width stretch of a 4
cm long uniform FBG when 250 mW of diode laser light
40 (power light 135) was directed through core 105 of multi -
mode fiber portion 100 (it was estimated that approximately
1 20 mW of power light 135 leaked) . FIG. 7, on the other hand,
shows a spectrum width compression of the 4 cm linear
chirped grating forming part of in- fiber optic component 120
45 when a 250 mW diode laser light was utilized as power light
135. In this situation, the temperature gradient created by
power light 135 “de-chirped” the linear chirped grating and
compressed the spectrum width.
According to a further alternate embodiment of the present
50 invention, shown in FIG. 8, a small local refractive index
change may be produced by generating a “hot spot” to modify
the reflection spectrum of a chirped FBG. Specifically, in-
fiber optical component 120 is, in this embodiment, a 4 cm
long linear chirped FBG (1 nm/cm), and thermal coating 115
55 is an approximately 2 mm long silver film. In one implemen-
tation investigated by the inventor, the power light 135 was
approximately 450 mW of 910 nm diode laser light. Power
light 135 heated thermal coating 115, which in turn heated
in-fiber optic component 120, thereby changing increasing
60 the local refractive index of the portion of in-fiber optic com-
ponent 120 near thermal coating 115 and expanding the grat-
ing period or periods. As a result, the resonance wavelength of
the in-fiber optic component 120 at this point increased. As
shown in FIG. 9, this localized heating created a notch filter in
65 the FBG reflection spectrum.
FIGS. 10A and 10B are side views of optical fiber system
140 according to still a further alternate embodiment of the
US 7,792,392 B2
11
present invention. As described below, optical fiber system
140 provides a dynamic Fabry-Perot micro-cavity resonator.
Fiber optical Fabry-Perot micro-cavity resonators are utilized
as an important optical component in fiber optic communica-
tion networks and fiber optic sensing systems. The present 5
invention, among other applications, may be utilized to either
stabilize the reflection spectrum of a Fabry-Perot micro-cav-
ity resonator from random temperature or stress-induced
drifting or to introduce a periodic phase change to the cavity
for phase-locking signal detection. As seen in FIGS. 10A and 10
10B, optical fiber system 140 includes first single mode fiber
portion 145 having core 150 and cladding 155. Optical fiber
system 140 also includes second single mode fiber portion
160 having a core 165 and cladding 170 that is joined to, such
as by fusion splicing, multimode fiber portion 190 having 15
core 195 and cladding 200. Optical fiber system 140 includes
an optical transducing element in the form of thermal coating
190 which is similar in structure to thermal coating 60 shown
in FIGS. 2 A and 2B. Thermal coating 190 is provided around
at least a part of, and preferably the entirety of, the circum- 20
ference of second single mode fiber portion 160.
As is known in the art, a Fabry-Perot filter is a high-spectral
resolution (narrow-band-pass) optical filtering device that
operates on the property of destructive light interference. A
Fabry-Perot filter includes a cavity bounded on each side by 25
two generally flat, transparent plates that have a partially
reflective coating provided thereon. Typically, the cavity is
filled with a dielectric material, which may include, without
limitation, air. Incident light is passed through the two coated
reflecting plates . The distance between the reflective coatings 30
determines which wavelengths will destructively interfere
and which wavelengths will be allowed to pass through the
coated plates. In addition, the optical transmission spectrum
of a Fabry-Perot filter typically shows multiple peaks with
narrow passband width. The spacing between neighboring 35
peaks is primarily determined by the gap between the two
reflecting plates that form the cavity and the dielectric func-
tion of the material present in the cavity. As seen in FIGS. 1 0A
and 10B, a partially reflective plate 215A is provided at the
end of the first single mode fiber portion 145 and a partially 40
reflective plate 215B is provided at the end of the second
single mode fiber portion 160 opposite partially reflective
plate 215A such that a cavity 220 is provided therebetween.
A sensing light 225 is directed through core 150 as shown
in FIGS . 1 0 A and 1 0B . Second single mode fiber portion 1 60 45
and multimode fiber portion 190 are joined to one another at
junction 205. As shown in FIGS. 10A and 10B, junction 205,
and in particular the different diameters of core 165 and core
195, act as an optical tap region that allows power light 230
that is directed through and propagates through core 195 to 50
leak out of core 195 and into cladding 170, where it is ulti-
mately absorbed by thermal coating 210. The absorbed power
light 230 heats thermal coating 210, which in turn heats
second single mode optical fiber 160. The heating of second
single mode fiber 160 causes its length to increase, thereby 55
decreasing the width of cavity 220 and changing the charac-
teristics of the Fabry-Perot filter implemented by partially
reflective plates 215A and 215B and cavity 220. In particular,
the addition of power light 230 will cause the reflection spec-
trum of sensing light 225 to be shifted as demonstrated in 60
FIG. 11.
Real-time gas and liquid flow sensing has many important
applications in, for example, aerodynamics, combustion
engine design, medical devices (such as respiratory devices)
and chemical analysis. At present, state-of-the-art flow sen- 65
sors are mostly based on MEMS technology. Although
MEMS -based devices have been found to be effective, the
12
packaging cost is relatively high, the packaged devices are
typically relatively bulky, and they rely on external electrical
power. As a result, the implementation of MEMS-based flow
sensors in small diameter flow tubes (as required in respira-
tory devices) and in harsh environments is currently not fea-
sible.
As an alternative, according to another aspect of the present
invention, a tunable (active) optical fiber system including an
FBG type in-fiber optic component powered by in- fiber light
such as is shown in FIGS. 2A and 2B or 4A and 4B may be
utilized to sense real-time gas and liquid (fluid) flow. In par-
ticular, if the FBG comprising in-fiber optic component 55
(FIGS. 2A and 2B) or in-fiber optic component 120 (FIGS.
4A and 4B) is heated as described herein such that the tem-
perature thereof is higher than the surrounding environment,
and if a gas or liquid is caused to flow past the associated
optical fiber 30 or 80, the thermal energy removed from the
FBG (in-fiber optic component 55 or 120) as represented by
the resulting temperature change will depend on the flow rate
of the surrounding gas or liquid. As a result, the flow rate can
be measured by measuring the resonance wavelength shift(s)
of the FBG that, as described above, are dependent upon FBG
temperature changes. Resonance wavelength shifts due to
fluid flow may be correlated to flow rates using known meth-
ods.
In one example implemented by the inventor, optical fiber
30 was provided with an in-fiber optic component 55 consist-
ing of a 5 mm uniform FBG having a resonance wavelength of
about 1553.7 nm at room temperature as shown in FIG. 12.
Power light 75 consisting of 384 mW diode laser light was
then provided, causing the resonance wavelength of the FBG
to shift to about 1558.2 nm as shown in FIG. 12. Air was then
caused to flow around optical fiber 30 at about 2.84 m/s,
which cooled optical fiber 30 down and removed heat from
the FBG, thereby causing another shift in the resonance wave-
length to about 1556.6 nm as shown in FIG. 12.
In another example implemented by the inventor, optical
fiber 30 was provided with an in- fiber optic component 55
consisting of a 5 mm uniform FBG having a resonance wave-
length of about 1554.9 nm at room temperature as shown in
FIG. 13. Power light 75 consisting of 384 mW diode laser
light was then provided, causing the resonance wavelength of
the FBG to shift to about 1558.2 nm as shown in FIG. 13. N 2
was then caused to flow around optical fiber 30 in a direction
perpendicular to the FBG at about 2.63 m/s, which cooled
optical fiber 30 down and removed heat from the FBG,
thereby causing another shift in the resonance wavelength to
about 1556.5 nm as shown in FIG. 13. FIG. 14 shows the
resonance wavelength shifts of the 5 mm uniform FBG as a
function of flow velocity using a power light 75 at two differ-
ent levels, 384 mW and 557 mW. As seen, the resonance
wavelength shifts closely follow a simple exponential decay
function of the N 2 velocity. Similar flow measurements were
also carried out on an in-fiber optic component 55 consisting
of a 1.7 cm uniform FBG having a resonance wavelength of
about 1538.3 nm at room temperature. As discussed else-
where herein, the magnitude of leaking power light 75 falls
off exponentially with distance from the junction 130,
thereby causing a temperature gradient in longer FBG such as
the 1.7 cm uniform FBG just described. FIG. 15 shows the
spectral evolution of such a 1 .7 cm uniform FBG heated with
a 442 mW power light 75 under N 2 flow velocities of 0.88 m/s
and 2.63 m/s. FIG. 16 shows the spectral width of the 7 cm
uniform FBG as a function of flow velocity using a power
light 75 at two different levels, 345 mW and 442 mW. As seen
in FIG. 16, the spectral widths decrease exponentially with
the increase in flow velocity.
US 7,792,392 B2
13
In addition, as will be appreciated, such a flow sensor may
be operated in a constant power mode or a constant wave-
length mode. In the constant power mode, a power light 75 or
135 having a constant power level is provided, and, as just
described, flow rate is measured based on resonance wave- 5
length shifts. In contrast, in the constant wavelength mode,
flow rate is measured based on the power level(s) of power
light 75 or 135 that is/are required to keep the resonance
wavelength of the FBG constant (equal to some pre-set, pre-
flow value) when liquid or gas flows thereby. FIGS. 17 and 18 to
demonstrate operation in the constant wavelength (variable
power) mode. Specifically, FIG. 17 shows the power levels of
power light 75 that are required to maintain a constant reso-
nance wavelength at various flow velocities with initial power
light levels of 1 52 mW and 335 mW using the 1 .7 cm uniform 1 5
FBG described above. Similarly, FIG. 18 shows the power
levels of power light 75 that are required to maintain a con-
stant resonance wavelength at various flow velocities with
initial power light levels of 249 mW, 348 mW and 538 mW
using the 5 mm uniform FBG described above. As seen in 20
FIGS. 17 and 18, the power levels needed to maintain a
constant, pre-set resonance wavelength follow linear func-
tions with the flow velocity.
As a further alternative, according to yet another aspect of
the present invention, a tunable (active) optical fiber system 25
including an FBG type in-fiber optic component powered by
in-fiber light such as is shown in FIGS. 2 A and 2B or 4 A and
4B may be utilized as a liquid level sensor to monitor the level
of a liquid in a container, such as, for example, and without
limitation, the level of liquid hydrogen in cryogenic fuel tanks 30
for space missions. One example of such an implementation
is shown in FIG. 19. As seen in FIG. 19, an optical fiber 80 as
described in connection with FIGS. 4 A and 4B is used to
monitor the level of water 250 contained in tank 255. Optical
fiber 80 includes single mode fiber portion 85 (having a core 35
and cladding (not shown)) that is joined to, such as by fusion
splicing, multi-mode fiber portion 100 (having a core and
cladding (not shown)) at a junction 130. Optical fiber 80 also
includes thermal coating 115 and an in-fiber optic component
(surrounded by thermal coating 115 and not shown in FIG. 40
19) in the form of a uniform 5 mm FBG. A sensing light 125
is directed through the core of single mode fiber portion 85
and a power light 135 (comprising ten watts of 910 nm laser
light from a high power diode laser array) is directed through
the core of multi-mode fiber portion 100. As described in 45
connection with FIGS. 4A and 4B, junction 130 acts as an
optical tap region that allows a portion of power light 135 to
leak out of the core and into of multi-mode fiber portion 100,
where it is ultimately absorbed by thermal coating 115. The
absorbed power light 135 heats thermal coating 115 which in 50
turn radiates heat that is conducted therefrom and heats in-
fiber optic component 120. FIG. 20 shows the spectral
response of the FBG of the optical fiber 80 of FIG. 19 in: (i)
water with no power light 135, (ii) in water with about 115
mW of power light 135, and (iii) in air with about 115 mW of 55
power light 135.
To determine whether the FBG is submersed inside the
water 250 inside tank 255 (level sensing), the 910 nm laser
was turned on to inject 115 mW power light into multi -mode
fiber portion 100 to heat the FBG. When the grating is sub- 60
mersed under the water 250, the resonance peak (solid trace)
of the heated FBG is shifted about 60-pm from the unheated
peak (dotted trace) . When the FBG is pulled above the level of
water 250, the FBG reflection peak rapidly shifted over about
1 .4 nm from 1541 nm to 1542.4 nm as shown in the FIG. 20. 65
This dramatic thermal response for the heated FBG thus
provides unambiguous detection whether or not the FBG is
14
immersed in the water 250. As will be appreciated, such an
optical fiber 80 may be used to determine whether a liquid in
a container such as tank 255 has fall below or risen above a
particular level by positioning the FBG at the level of interest,
heating the FBG with a power light 135, and monitoring the
spectral response of the FBG.
The thermal responses of a heated grating, such as the FBG
of optical fiber 80 shown in FIG. 19, in air, water, and liquid
nitrogen as a function of input laser power (power light) are
characterized in FIG. 21. As expected, the heated grating
when exposed to air produced the largest resonance wave-
length shift. The peak shift of the heated grating in ambient
room temperature air follows a linear variation with input
laser power with a slope of 15 pm per mW. A 10-mW laser
input will produce a 150 pm reflection peak shift. This is in
contrast to a 20 pm shift in water and a five pm shift in liquid
nitrogen. The grating wavelength shifts in water and liquid
nitrogen were re-plotted using a reduced vertical scale as
shown in the inlet of FIG. 21.
FIG. 22 is an alternate embodiment of an optical fiber 80
utilized as a liquid level sensor. The optical fiber of FIG. 22 is
similar to the optical fiber 80 of FIG. 19 except that it includes
four thermal coatings 115 (each surrounding a 5 mm uniform
FBG) spaced 3 cm apart from one another along single mode
fiber portion 85. For convenience, the FBGs shall be referred
to as sensor 1, sensor 2, sensor 3 and sensor 4, with sensor 4
being located about 3 cm from junction 130. As seen in FIG.
22, sensor 1 has the shortest resonance wavelength and is the
topmost FBG, and sensor 4 has the longest resonance wave-
length is the lowest FBG. The optical fiber 80 of FIG. 22 can
thus be used to sense the presence of liquid at four different
locations, and as a result can sense four different liquid levels
within a container such as tank 255.
FIGS. 23 A through D show the reflection spectrum of each
FBG (sensors 1-4) in an unheated condition (no power light
135) in dotted line form. When the optical fiber 80 is pulled
out from the water 250, sensor 1 is the first grating to rise
above the water level and sensor 4 is the last to emerge. FIG.
23A shows the reflection spectrum of the heated FBG when
sensor 1 is pulled out from the water with 600-mW input laser
power (power light 135). The resonance peak for sensor 1
shifted 350 pm from 1535.7 nm to 1536.05 nm. Due to the
much larger specific heat and thermal convection rates of
water than those of air, reflection peaks for heated gratings
remaining in the water shifted less than 1 0 pm. The dramatic
resonance peak shift for sensor 1 above the liquid surface
provides unambiguous detection of the liquid level. As shown
in FIGS. 23B-23D, similar behavior is observed when sensors
2, 3 and 4 are pulled out from the water 250 in Succession.
FIGS. 23Cand23D also show non-uniform peak shifts for the
different FBGs while the input laser power (power light 135)
is reduced to 550 mW and then to 400 mW. This is due to the
non-uniform leakage profile in single mode fiber portion 85.
The power is reduced to avoid the spectral deformation of
sensor 4 due to overheating.
The level sensing applications have been described herein
using optical fiber 80 as shown in FIGS. 4 A and 4B. It should
be understood, however, that this is for illustrative purposes
and that other embodiments of the present invention, such as
the optical fiber 30 shown in FIGS. 2A and 2B may also be
used in level sensing applications.
Finally, the particular embodiments described above in
connection with FIGS. 2-12 have been based on the conver-
sion of the in-fiber power light (75, 135, 230) to thermal
energy using an optical transducing element comprising a
light absorbing thermal coating (60, 115, 210). However, the
concept of the present invention is not limited to the conver-
US 7,792,392 B2
15
sion of the in-fiber power light to thermal energy. The in-fiber
power light may also be converted to other energy types (that
are then used to tune an in-fiber optic component), such as
mechanical, acoustic, electrical, magnetic and optical (at
other wavelengths) energy using various types of alternative
transducing elements and energy conversion/harvesting
mechanisms. For example, recent developments in the area of
photo-mechanics have shown that polymer membranes con-
taining light-sensitive molecules undergo rapid photo-con-
traction or expansion under weak polarized light radiation. In
particular, recent investigations have shown that liquid-crys-
tal membranes containing azobenzene chromeophore can be
repeatedly bent without apparent fatigue. Such membranes
may be provided on an optical fiber containing an in-fiber
optic component, and bending, twisting, stretching and/or
compressing of such membranes using in-fiber power light
may be utilized to tune the in-fiber optic component (e.g., to
change the spacing of the grating of an FBG or to deform a
micro-mirror to adjust the Q-value and finesse of a micro-
optical resonator). In this case, the membrane acts as an
on-fiber actuator. In addition, such a membrane may be
attached to a piezo actuator membrane to provide on-fiber
optical-to -electrical conversion, without wires attached from
the light transmitting end of the fiber. In addition, laser micro-
machining of a thermal coating such as thermal coatings 60
and 115 will enable in- fiber optical energy to induce periodic
index modulation to produce long period fiber grating filters
for in-fiber power equalization.
FIG. 24 is a side isometric view (in partial cross-section) of
a hydrogen sensor 300 according to a further embodiment of
the present invention. As seen in FIG. 24, the hydrogen sensor
300 includes an optical fiber 305 that includes a core 310, an
inner cladding 315, an outer cladding 320 and a protective
layer 325. Preferably, and as described elsewhere herein in
connection with the optical fiber 30 shown in FIGS. 2A and
2B, the core 310, inner cladding 315 and outer cladding 320
are made of light propagating materials, wherein the core 310
has an index of refraction that is greater than the index of
refraction of the inner cladding 315, which in turn is greater
than the index of refraction of the outer cladding 320. Except
as otherwise described herein, establishing the relative indi-
ces of refraction in this manner causes light propagating in
core 310 to be confined therein, and light propagating in inner
cladding 315 to be confined therein.
In one particular embodiment, the core 310 is made of a
glass material such as fused silica that is doped with germa-
nium and/or boron to increase the index of refraction thereof,
the inner cladding 315 is made of fused silica, and the outer
cladding 320 is made of fused silica that is doped with fluo-
rine to decrease the index of refraction thereof. Particular
dopant levels and indices of refraction may be as described
elsewhere herein, such as in connection with the optical fiber
30 shown in FIGS. 2A and 2B. Alternatively, outer cladding
320 may be made of a polymer material such as clear silicone
or PFA.
As seen in FIG. 24, the optical fiber 305 includes an FBG
330 written into the core 310 in a suitable manner, such as,
without limitation, any of the manners described elsewhere
herein. Alternatively, another type of wavelength resonant
in-fiber optic component (i.e., a component that reflects par-
ticular wavelengths dependent on the characteristics of the
component), such as, without limitation, a Fabry -Perot filter,
may be employed instead of FBG 330. In addition, a palla-
dium layer 335 is provided around at least a portion of and
preferably the entirety of the outer circumference of the outer
cladding 320 (the protective layer 325 has been removed at
this location) in a position that is proximate to the position of
16
the FBG 330 (FIG. 24 shows thermal coating 60 in partial
cut-away to enable viewing of the other components of opti-
cal fiber 30). The palladium layer 335 is a layer of material
that includes palladium, and may be palladium only or a
5 palladium alloy. The palladium layer 335 acts as a hydrogen
absorbing material for the reasons described below. As is
known, when palladium is in the presence of ambient hydro-
gen, a chemical reaction occurs wherein palladium hydride is
formed. In addition, during this reaction, the material
1 0 expands . As a result, in the presence of ambient hydrogen, the
palladium layer 335 will form palladium hydride and will
expand and thereby induce strain in the optical fiber 305 at a
location adjacent thereto. The magnitude of the expansion
and thus the strain is dependent upon the amount of hydrogen
15 that is absorbed. As is also known, palladium does not nor-
mally adhere well to glass materials. Thus, in the preferred
embodiment, an intermediate layer 340 that adheres well to
both glass materials and palladium, such as a glue metal, is
first provided around at least a portion of and preferably the
20 entirety of the outer circumference of the outer cladding 320,
and then the palladium layer 335 is provided over the inter-
mediate layer 340 as shown in FIG. 24. The palladium layer
335 and the intermediate layer 340 may be applied by any one
of many known coating methods such as sputter coating, and
25 may be thickened economically by electro or electroless plat-
ing.
Referring to FIG. 24, the optical fiber 305 also includes an
optical tap region 345 located in a portion of the optical fiber
3 05 that is proximate to the palladium layer 335 . As described
30 elsewhere herein, the optical tap region 345 is a region of the
optical fiber 305 that will allow certain light, as described in
greater detail below, that is propagating through the optical
fiber 3 05 to leak out of (i.e., be released from) the optical fiber
3 05 and be absorbed by the palladium layer 335 . As described
35 elsewhere herein, the optical tap region 3 45 may be created in
a number of ways. For example, laser techniques or ion-
implantation techniques may be used to, in effect, damage the
inner cladding 315 in a selected region and thereby alter its
index of refraction such that the power light 355 (described
40 below) will leak out of the inner cladding 3 1 5 at the optical tap
region 345.
In operation, as illustrated in FIG. 24, a sensing light 350 is
directed through and propagates through the core 310. As is
known in the art and as described elsewhere herein, the sens-
45 ing light 350 will propagate through the core 310 and encoun-
ter the FBG 330, and a particular resonance wavelength will
be reflected back in the opposite direction. In addition, as
described above, the palladium layer 335 absorbs any ambi-
ent hydrogen and is caused to expand a certain amount
50 depending upon the amount (concentration) of hydrogen that
is present. The expansion of the palladium layer 335 will
induce a strain in the optical fiber 305, which strain will
stretch the core 310 (and the entire fiber) and thereby change
the spacing of the gratings of the FBG 330. As also described
55 elsewhere herein, the particular resonance wavelength that is
reflected by an FBG depends upon the grating spacing. Thus,
changes in the spacing of the gratings of the FBG 330 result-
ing from strain induced by the expansion of the palladium
layer 335 will change the resonance wavelength that is
60 reflected by the FBG 330. The spectral shifts (changes in the
resonance wavelength that is reflected by the FBG 330) can
thus be monitored and used to characterize and determine the
ambient hydrogen content in the area proximate to the FBG
330 and palladium layer 335. However, as described in the
65 “Background of the Invention” section herein, as the ambient
temperature decreases (e.g., below, for example, a range of
about 20-50 degrees C.), palladium absorbs hydrogen
US 7,792,392 B2
17
increasingly slowly, and, as a result, and without the improve-
ment of this embodiment of the present invention described
below, sensitivity would be low and response time would be
increasingly slow (below one minute, up to hours or days) at
any hydrogen concentration at such low temperatures, and in
particular at relatively low hydrogen concentrations (e.g.,
below 4%).
Thus, according to an aspect of this embodiment of the
present invention, a power light 355 is simultaneously
directed through both the core 310 and the inner cladding 315
(or, alternatively, in just the inner cladding 315). Although the
power light 355 is shown propagating in a direction opposite
the sensing light 350, it may also be propagated in the same
direction as sensing light 350. The power light 355 may be,
for example, light generated from a high-power diode laser
array (not shown). The power light 355 will, as seen in FIG.
24, propagate through the core 310 and the inner cladding 315
and will be confined therein until it reaches the optical tap
region 345. When the power light 355 reaches the optical tap
region 345, at least a portion of the power light 355 will leak
out of the inner cladding 315 and into the outer cladding 320.
The portion of the power light 355 that has leaked into the
outer cladding 320 will then be transmitted substantially radi-
ally outwardly therefrom and will ultimately be absorbed by
the palladium layer 335. The absorption of power light 355 by
the palladium layer 335 induces heating of the palladium
layer 335, which increases the temperature of palladium film
and in turn increases the hydrogen absorption rate (and thus
decreases the hydrogen absorption time) of the palladium in
the palladium layer 335. As a result, the sensitivity and the
response time of the hydrogen sensor 300 are increased, par-
ticularly at lower temperatures. In another embodiment, at
least a portion of the power light may be absorbed by another
part of the hydrogen sensor 300, such as a portion of the inner
cladding 315 or the outer cladding 320. When the power light
is so absorbed, heat will be generated which is then conducted
to the palladium layer 335 to heat the palladium layer 335 and
thereby increase the gas absorption rate.
Thus, as just described, the propagation of the power light
355 within the optical fiber 305 improves the performance of
the hydrogen sensor 300 by heating the palladium layer 335.
In one embodiment, the propagation of the power light 355 is
continuous, thereby providing continuous heating of the pal-
ladium layer 335 . However, as is known, the reaction between
hydrogen and palladium that produces palladium hydride and
expansion of the palladium layer 335 is relatively fast, on the
order of a few seconds. Thus, while the continuous heating
method described above will be effective, it will result in
more power being used than is necessary as the heating is not
required once the chemical reaction is complete. Thus
according to another embodiment of the invention, a flash
heating method is employed to improve the performance of
the hydrogen sensor 300 at low temperatures (as described
elsewhere herein) wherein power is conserved. That method
is shown in the flowchart shown in FIG. 26.
Referring to FIG. 26, the method begins at step 400,
wherein the sensing light 350 is directed through the core 310
of the hydrogen sensor 3 00 at the ambient temperature (which
may be a relatively low temperature on the order of -20
degrees C. or less) and the resonance wavelength that is
reflected by the FBG 330 is measured. As described else-
where herein, the hydrogen absorption by the palladium layer
335 at such low temperatures will be slow. Next, at step 405,
the power light 355 is simultaneously directed through both
the core 310 and the inner cladding 315 (or, alternatively, in
just the inner cladding 315) for a defined period of time and
then is shut off. During that time, when the power light 355
18
reaches the optical tap region 345, at least a portion of the
power light 355 will leak out of the inner cladding 315 and
into the outer cladding 320 . The portion of the power light 355
that has leaked into the outer cladding 320 will then be trans-
5 mitted substantially radially outwardly therefrom and will
ultimately be absorbed by the palladium layer 335, thereby
heating the palladium layer 335 and increasing the hydrogen
absorption rate. The amount of time that the power light 355
is propagated is preferably just long enough for the reaction
to between the ambient hydrogen and the palladium in the pal-
ladium layer 335 to be completed (producing palladium
hydride). That time may be, for example and without limita-
tion, on the order of 1 0 seconds. Next, at step 410, the hydro-
gen sensor 300 is allowed to cool back down to ambient
15 temperature. As will be appreciated by those of skill in the art,
following the cool down, absorbed hydrogen is “locked in”
the palladium layer 335 in the form of the palladium hydride.
In other words, the cool down to ambient will not cause the
reaction to be reversed. Then, at step 415, the sensing light
20 350 is once again directed through the core 310 of the hydro-
gen sensor 300 at the ambient temperature (it may be left on
or turned off after step 400 and on again in this step) and the
resonance wavelength that is reflected by the FBG 330 is once
again measured. At step 420, the shift in the measured reso-
25 nance wavelength that is reflected by the FBG 330, if any, is
determined. As described elsewhere herein, if ambient hydro-
gen is present, its absorption by the palladium layer 335 will
cause the palladium layer 335 to expand, thereby inducing a
strain in the FBG 330 and a resulting resonance wavelength
30 shift. Finally, at step 425, the ambient hydrogen concentration
is determined based on the resonance wavelength shift (be-
fore and after the heating) due to the hydrogen absorption
induced strain.
In addition, the hydrogen sensor 300 can be returned to its
35 original, baseline form, so that it can be used again to make
another measurement, by heating the palladium layer 335
using the power light 355 to an extent sufficient to “de-gas”
the absorbed hydrogen (i.e., reverse the chemical process to
convert the palladium hydride back to palladium and hydro-
40 gen). Once this is done, the palladium layer 335 will return to
substantially its original size, thereby removing the hydrogen
absorption induced strain and returning the spacing of the
gratings of the FBG 330 to their original size (i.e., the FBG
330 will return to its original length). As a result, the reso-
45 nance wavelength will return to its baseline level.
FIG. 25 is a side isometric view (in partial cross-section) of
a hydrogen sensor 300' according to an alternate embodiment
of the present invention. The hydrogen sensor 300' is similar
to the hydrogen sensor 300 except that, as seen in FIG. 25, in
50 the hydrogen sensor 3 00', a portion of the outer cladding 320
is removed and the intermediate layer 340 and the palladium
layer 335 are applied around the inner cladding 315. Since a
portion of the outer cladding 320 is removed, the power light
355 will be permitted to leak out of the inner cladding 315 at
55 that location and will be absorbed by the palladium layer 335
to provide the effect described above. The hydrogen sensor
300' may be used with either of the methods described herein
(continuous heating or flash heating).
In one particular embodiment of the hydrogen sensor 300',
60 the core 3 1 0 of the optical fiber 305 was a standard 8 pm fiber
core with a N.A. of 0.12, and the outer cladding was a low-
index plastic jacket having a N.A. of 0.48 to provide confine-
ment of propagation of the power light 355 in the inner
cladding 315. The protective coating 325 was an acrylate
65 coating. To inscribe the FBG 330, the optical fiber 305 was
first photosensitized via hydrogen loading for 1 week at 120
bars. After stripping a portion of the protective coating 325
US 7,792,392 B2
19
and the outer cladding 320 , a one-centimeter FBG 330 was
written into the core 310 of the fiber 305 using a phase mask
and a pulsed UV Excimer laser. Following grating fabrica-
tion, the fiber 305 was loaded into a magnetron sputtering
machine. A fiber sputtering mount which allowed rotation of
the fiber 305 during sputtering deposition was utilized to
provide a uniform axial coating. To improve adhesion of the
palladium forming the palladium layer to the silica surface of
the inner cladding 315 , a 20 nm layer of a glue metal was
applied to the fiber 305 (to form the intermediate layer 340 )
followed by the much thicker (150-500 nm) palladium layer
335 .
Testing of the hydrogen sensor 300 * described above was
performed in a sealed temperature-regulated chamber
capable of maintaining -120-120° C. through the use of
encapsulating liquid nitrogen cooling or recirculating fluid
heating. Fiber feed-throughs were placed at either end of the
chamber for sensor access. Two mass flow controllers pro-
vided a variable hydrogen and nitrogen mix for 0-10% H 2 .
Chamber pressure, temperature, and gas flow were monitored
continuously.
The hydrogen sensor was then connected for testing and
evaluation. The experimental setup can be found in K. P.
Chen, L. J. Cashdollar, “Controlling Fiber Bragg Grating
Spectra with In-Fiber Diode Laser Light,” IEEE Phot. Tech-
nol. Lett., vol. 16, pp. 1897-1899, (2004), and id briefly
described here. A high-power 910 nm diode laser (Qphoton-
ics QLDM-910-1 .5) was used to optically heat the fiber sen-
sor. The diode laser is pigtailed with a 110-|im core multi-
mode fiber. One end of the double-clad fiber containing the
sensor was fusion spliced to the multimode fiber output of the
diode laser. The other end of the double-clad fiber was spliced
to a recirculator with a broadband light source, and a spec-
trum analyzer (Ando 6317B) for FBG sensor monitoring.
FIG. 27 shows the response of the Bragg wavelength to
various hydrogen concentrations at room temperature. These
measurements were performed for both 350-nm and 1 50-nm
Pd coatings. The measurement was taken after 30 seconds of
exposure to hydrogen. The thicker 350 nm Pd coated grating
exhibits 0.37 nm response for 10% hydrogen, which shows
42.3% better response than 1 50-nm coating’s response of
0.26 nm. The lowest tested hydrogen concentration was
0.5%, yielding 15 pm shift in the 350-nm Pd-coated FBG.
The measurement was repeatable and showed little hysteresis
at room temperature and low hydrogen concentrations. The
sensor was also tested at the optimal temperature of 150° C.,
in which the 350-nm coated Pd-FBG shows a dramatic
improvement in sensitivity and response time. The response
time therein was found to be less than 9 seconds (instrument-
limited). By replacing the broadband source and spectrum
analyzer with a tunable laser, 0.125% hydrogen could be
readily detected at 150° C.
FIG. 28 explores the inherent problem associated with H 2
sensing at low temperatures. Herein, the 350 nm Pd coated
sensor is exposed to 10% H 2 as the temperature is slowly
raised while pausing for 30 s at each successive 10 degree
increment and measuring the Bragg wavelength. The same
test is repeated without hydrogen, and the two responses are
compared. The response due to the presence of hydrogen is
imperceptible until the temperature reaches about -20° C.
This is because the absorption rate of hydrogen is extremely
low at low temperatures.
This problem can be readily solved by the active fiber
sensor technology described herein and is demonstrated in
FIGS. 29 A and 29B. In a -50° C. environment, a passive
350-nm Pd-FBG sensor does not respond to 1 0% hydrogen in
a reasonable time frame. However, since it is known that
20
Pd-FBG sensors respond well at room temperature, accord-
ing to the present invention, high-power diode laser light can
be used to locally heat the FBG sensor to a temperature region
in which sensors have reasonable responsivity. This is accom-
5 plished, in one embodiment, by stripping away the double
clad fiber’s outer coating; and re-coating with the Ta/Pd lay-
ers that serve to absorb the power light traveling in the inner
cladding. This local sensor heating facilitates rapid hydrogen
gas diffusion and reaction with the Pd film to form hydrides as
to shown in FIG. 29A. The heated FBG shows some spectral
chirp, probably due to the non-uniform heating profile. The
injection of 560 mW of laser power shifted the -50° C. FBG
wavelength up from 1545.65 mil to 1546.45 nm, which cor-
responds to a sensor temperature of ~+10° C., determined
15 from FIG. 28. Using a cut-back technique, it was estimated
that 53% laser power was absorbed by the 2-cm long coating.
After a brief 10-second laser heating, the laser is abruptly
turned off and the sensor is allowed to cool rapidly to the
ambient temperature, and the absorbed hydrogen is locked
20 temporarily into the Pd film. A wavelength shift due to the H 2
absorption induced strain can be used to gauge the hydrogen
concentration. At -50° C. the 350-nm Pd coated FBG shows
0.25 nm shift for 10% H 2 in FIG. 29A, which is approxi-
mately 67% of that at room temperature. This reduced
25 responsivity is probably due to the FBG having a smaller
thermo -optical coefficient at low temperatures. FIG. 29B
serves as a comparison for this heating cycle, in which the
same procedure is. performed without the presence of hydro-
gen. Herein, the same thermal shift is noted due to the laser
30 heating. However, after the heating is removed, the Bragg
wavelength returns to its original position at the -50° C.
ambient with no noticeable shift or deformation of the FBG
reflection spectrum.
FIG. 30 illustrates the overall success of low temperature
35 hydrogen leak detection using the in-fiber laser heating tech-
nique of the present invention. In this experiment, the ambient
temperature is set at -50° C. while the hydrogen concentra-
tion is gradually increased. At each measured concentration,
the heating laser is cycled on for 30 seconds, turned off, and
40 the sensor response is measured. Full-scale response at -50°
C. was approximately 50% to that at the room temperature,
and the shape of the response curve is altered somewhat. In
general, it was shown that relatively fast detection at low
temperatures is possible using the laser heating technique of
45 the present invention.
Thus, the experimental work described herein demon-
strates an effective means to enhance the low-temperature
performance of Pd-FBG sensors using the in-fiber optical
heating technique of the present invention. By locally con-
50 trolling the adsorption and degassing temperature of a Pd
film, it is possible to shorten response time over a range of
temperatures, thus facilitating fast operation of the hydrogen
sensor at all temperatures. The technique described herein
facilitates the construction of a FBG hydrogen sensor array
55 with only one fiber and one feed-through. The elimination of
electrical wires and the electricity traditionally used to heat
the fiber sensor preserves all of the intrinsic advantages of
fiber sensors and dramatically simplifies device packaging.
While specific embodiments of the invention have been
60 described in detail, it will be appreciated by those skilled in
the art that various modifications and alternatives to those
details could be developed in light of the overall teachings of
the disclosure. For example, although two embodiments of a
hydrogen sensor have been shown and described herein, it
65 will be appreciated that the present invention may be utilized
to detect gasses other than hydrogen, in which case the pal-
ladium layer would be replaced by a gas absorbing material
US 7,792,392 B2
21
22
that experiences increased absorption of the gas in question
when heated by the power light as described herein. Accord-
ingly, the particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of the inven-
tion which is to be given the breadth of the claims appended
in any and all equivalents thereof
What is claimed is:
1. A method of sensing a gas at an ambient temperature,
comprising:
providing an optical fiber, wherein said optical fiber has a
core, a wavelength resonant in-fiber optic component
provided in said core at a first location, and at least one
layer of a material attached to said optical fiber in prox-
imity to said first location, said material being able to
absorb said gas at a temperature dependent gas absorp-
tion rate, said gas absorption rate increasing when a
temperature of said material is increased;
propagating a first sensing light in said core at said ambient
temperature, said wavelength resonant in-fiber optic
component receiving said first sensing light and reflect-
ing a first reflected light having a first resonance wave-
length that is dependent on an ambient characteristic of
said wavelength resonant in-fiber optic component;
propagating a power light in said optical fiber for a defined
period of time during which at least a portion of said
power light is used to heat said material and cause said
material to induce a strain in said optical fiber, said strain
changing the ambient characteristic to a changed char-
acteristic;
allowing said material to cool to a temperature substan-
tially equal to said ambient temperature after said
defined period of time has expired;
propagating a second sensing light in said core after said
material is allowed to cool, said wavelength resonant
in-fiber optic component receiving said second sensing
light and reflecting a second reflected light having a
second resonance wavelength that is dependent on said
changed characteristic of said wavelength resonant in-
fiber optic component;
determining a difference between said first resonance
wavelength and said second resonance wavelength; and
using said difference to determine at least one of a presence
of and a concentration of said gas.
2. The method according to claim 1, wherein said propa-
gating a power light comprises propagating a power light in
said optical fiber for a defined period of time during which at
least a portion of said power light is released from said optical
fiber at said first location and absorbed by said material,
wherein said absorbed at least a portion of said power light
heats said material and causes said material to induce a strain
in said optical fiber, said strain changing the ambient charac-
teristic to a changed characteristic.
5 3. The method according to claim 1, wherein said propa-
gating a power light comprises propagating a power light in
said optical fiber for a defined period of time during which at
least a portion of said power light is absorbed at or about said
first location, wherein said absorbed at least a portion of said
to power light heats said material and causes said material to
induce a strain in said optical fiber, said strain changing the
ambient characteristic to a changed characteristic.
4. The method according to claim 1, wherein said wave-
length resonant in-fiber optic component is a fiber Bragg
15 grating and wherein said ambient characteristic is a grating
spacing of said fiber Bragg grating at said ambient tempera-
ture and wherein said changed characteristic is a grating
spacing of said fiber Bragg grating following said step of
allowing said material to cool.
20 5. The method according to claim 1, wherein said wave-
length resonant in-fiber optic component is a Fabry-Perot
filter and wherein said ambient characteristic is a length of at
least a portion of said Fabry-Perot filter at said ambient tem-
perature and wherein said changed characteristic is a length
25 of at least a portion of said Fabry-Perot filter following said
step of allowing said material to cool.
6. The method according to claim 1, wherein said gas is
hydrogen and wherein said material includes palladium.
7. The method according to claim 6, wherein said material
30 is palladium.
8. The method according to claim 6, wherein said material
is a palladium alloy.
9. The method according to claim 6, wherein during said
defined period of time a reaction occurs between said hydro -
35 gen and said palladium to produce palladium hydride, and
wherein said reaction causes said at least one layer of a
material to expand and induce said strain.
10. The method according to claim 9, wherein said defined
period of time is long enough to allow said reaction to be
40 completed.
11. The method according to claim 1, wherein said first
sensing light and said second sensing light are portions of a
continuously propagated light.
12. The method according to claim 1, wherein the step of
45 propagating the first sensing light ends prior to said allowing
step commencing and the step of propagating the second
sensing light begins after said allowing step is completed.
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